Method for depositing thin films



y 1951 c. H. ALEXANDER ETAL 2,553,289

uz'mon FOR DEPOSITING THIN FILMS Filed March 12, 1947 CH. ALEXANDER C. A. BAER Zmventors (Ittomeg Patented May 15, 1951 METHOD FOR DEPOSITING THIN FILMS Collin H. Alexander and Charles A. Baer, Rochester, N. Y., assignors to Bausch & Lomb Optical Company, Rochester, N. Y., a corporation of New York Application March 12, 1947, Serial No. 734,020

4 Claims. 1 This invention relates to the deposition of thin lms of materials on a surface of a body in a vacuum and more particularly it has reference.

to a method for treating the surface prior to, during, and after deposition of the material.

Thin films of various kinds'of materials, such as aluminum, rhodium, magnesium fluoride, cryolite, and zinc sulfide, are now Widely used in the optical industry. Metallic films are deposited on glass or plastic surfaces to provide mirrors while thin layers of magnesium fluoride, cryolite, and zinc sulfide are used on the surfaces of optical elements in order to decrease the reflection of light and thereby increase the transmission of light through the elements. It is necessary that the surfaces be treated before the deposition of such materials in high vacuum in order to remove the adsorbed gases, water vapor, and other material in order to provide a durable, tenacious film. Thi treatment is generally acccmplished either by subjecting the surface to a glow discharge in a vacuum having a pressure greater than about 10 microns or by heating the surface by means of suitable heating elements positioned within the evacuated chamber. Another method of treating the surfaces prior to the deposition of the material involve the bombardment of the surface by a stream of electrons which is carried out in a vacuum under pressures of about 0.01-10 microns as disclosed in application for U. S. patent, Serial No. 555,097, filed by Richard M. Rice, and issued on May 20, 1947 as Patent No. 2,420,724.

When the glow discharge method are used,

some of the positive ions collide with the metal parts within the chamber and so act to sputter off some of the metallic material which then becomes deposited on the surface which is being treated so that the Subsequent deposited layer of material exhibits an undesirable graininess. the pressures are reduced, the amount of such sputtering is diminished. With the electronic bombardment method as described in said application, some sputtering also occurs so that the final layer of material has a grainy structure. Such a grainy structure is, of course, undesirable in the case of a mirror because the graininess tends to scatter the light and so affects the image forming characteristics of the mirror.

The sputtering action which is present with electronic bombardment methods is due to the fact that an electron may collide with a wandering gas molecule and form a positive ion which would be attracted to the metal table or other negatively charged metal parts in the chamber 2 and so act to sputter ofl material from these parts.

Accordingly, it is an object of this invention to provide a method whereby the foregoing disadvantages may be eliminated when a surface'is treated by electronic bombardment prior to the deposition of material thereon. A further object is to provide an improved method for depositing thin films cf material on a surface of a body in a vacuum. 7 Another object is to provide a method whereby improved films of material may be deposited on a commercial scale on var,- ious types of surfaces in a vacuum. These and other objects and advantages will be more fuliy described and pointed out in the appended claims. j

The drawing is a diagrammatic view of an apparatus which can be used for practising our method.

Our method may be carried out by the apparatus shown in the drawing wherein a metal base plate H) supports the glass bell jar H with the juncture therebetween sealed by suitable vacuum-tight gasket means l2. Mounted within the chamber is the work support which comprises a metal plate I3 having seats M for holding the work such as lenses l5. The plate I3 is supported by metal rods l6 which are electrically insulated from the plate ID by means of the insulators H. The bell jar may be evacuated by means of a vacuum pump P connected to the chamber by means of conduit Ill. 7

A suitable heating coil l8 having leads :9 passing through insulating bushings 20 in plate In is adapted to support the material which is to be vaporized, such as the loops of aluminum wire 2 I. The leads iii are connected to a suitable source of electric current so that the material 2| may be heated and vaporized.

In order to provide a source of electrons in the chamber for the bombardment of the lens surfaces prior to the evaporation of the material, there is provided a tungsten wire filament 22 which may be heated by a suitable electric current passing through leads 23 which are insulatedfrom the plate I0 by bushings 24. A high voltage transformer 25 has one of its secondary terminals connected with the rod 1 B by means of a lead 26 passing through the bushing 21 in plate 10. The other secondary terminal is connected to one of the leads 23 which supply current to the electron emitting filament 22. The transformer or rectifier 25 delivers from 2,000-15,000 volts pulsating filtered D. C. or A. C. The positive terminal is connected to the rod I 6 so that the work holder l3 and the lenses are at a positive potential. The base plate I8 is connected to the ground by lead 28. A cylindrical metal shield 29, supported on rods 39 and insulated from the base ill by insulators 3|, prevents the deposition of material on the rods 16 and bushings l1.

In one method of practising our invention, the lenses l5 are mounted on the work holder 13 and the chamber is evacuated to a pressure of about 0.1 micron of mercury. The transformer is energized so that a potential of about 2,000- 15,000 volts is applied to the Work holder [3. The electron emitting filament 22 is also energized so that electrons therefrom bombard the surfaces of the lenses l5. After the filament 22 has been energized for about eight to ten minutes, the filament i8 is energized so as to evaporate the material 2i.

sired thickness on the lenses it, the current is shut off from the filament [8 but the emission of electrons from the filament 22 is preferably continued for two or three minutes longer. All current supplies are then disconnected, the vacuum released, and the coated lenses removed.

It will thus be apparent that the electrons which are emitted by the filament 22 under high velocity will pass upwardly to the work holder l3 and the lenses 15. If an electron collides with a gas molecule and forms a positive ion, the latter will be attracted to the filament 22 since it is the only negatively charged part in the chamber when D. C. is used. The electrons from the filament 22 will collide with the positive ions and will drive them away from the filament or will combine with them and form a gas molecule which, having no charge, wanders off into the system. When A. C. is used, the plate 43 is positive for a half cycle during which electrons from filament 22 pass towards the plate l3; During the other half cycle, the plate l3, being negative, does not attract the electrons from the filament 22 and hence there is less chance for collisions between electrons and gas molecules and so less chance for the formation of positive ions which would cause sputtering. Since the positive ions will thereby be substantially prevented from striking the exposed metal parts in the chamber, there will be little or no sputtering r of such metal parts and hence such metal will not be deposited on the lens surfaces. As a consequence, the layer of material which is subsequently evaporated and condensed onto the lens surface [5 will be homogeneous in character and substantially free from the undesired grainy structure.

Our improved method has been used successfully in the commercial production of metallic mirrors which'are not only hard and durable but are also free from graininess so that they will provide specular reflection of light rays rather than diffused reflection. Such types of aluminum mirrors have, mercially used in television sets. Our method has also been successfully used for the produc- When the'materialhas been evaporated and condensed to the defor example, been com-.

tion, on a commercial basis, of anti-reflection films of magnesium fluoride, cryolite, and zinc sulfide on lens surfaces.

From the foregoing, it will be apparent that .we are able to attain the objects of our invention and provide an improved method for depositing thin films ofmaterial on surfaces of a body in a vacuum. By means of our method, durable, corrosion-resistant films may be deposited and such films will have the further advantage that their structure is more homogeneous than that of the films deposited by prior art methods. Various modifications can obviously be made without departing from the spirit of the inventionas pointed out in the appended claims.

We claim:

1. A method of depositing a film of material on a surface of a body in an evacuated chamber having metal parts exposed therein which comprises the steps of bombarding the surface with electrons from a source, maintaining the metal parts at a potential which is positive relative to said source during the bombardment of the -,surface whereby positive. ions formed in the chamber are substantially prevented from strik ing the metal parts and depositing a film of material on the surface by thermal evaporation immediately after the bombardment of the sur face.

2. A method of depositing a thin film of material on an optical surface of a body in an evacuated chamber having metal parts exposed therein which comprises the steps of bombarding the surface with a stream of electrons from a source while maintaining the pressure in the chamber at about 0.01-10 microns and maintaining said metal parts at a potential which is positive relative to the potential of the source whereby positive ions formed in the chamber will be substantially prevented from sputtering material from said parts onto the surface and thereafter depositing the film of material on the surface by thermal evaporation in the evacuated chamber.

3. The invention as set forth in claim 2 Wherein the thin film of material is formed of ma nesium fluoride.

4. The invention as set forth in claim 2 wherein the thin film of material is formed of aluminum.

COLLIN H. ALEXANDER. CHARLES A. BAER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,256,771 Berghaus et al. Sept. 23, 1941 2,383,469 Colbert Aug. 28, 1945 2,391,595 Richards et al Dec. 25, 1945 2,420,724 Rice May 20, 1947 2,447,979 Hansel et al. Aug. 24,1948 2,456,708 Kellogg Dec. 21, 1948 

1. A METHOD OF DEPOSITING A FILM OF MATERIAL ON A SURFACE OF A BODY IN AN EVACUATED CHAMBER HAVING METAL PARTS EXPOSED THEREIN WHICH COMPRISES THE STEPS OF BOMBARDING THE SURFACE WITH ELECTRONS FROM A SOURCE, MAINTAINING THE METAL PARTS AT A POTENTIAL WHICH IS POSITIVE RELATIVE TO SAID SOURCE DURING THE BOMBARDMENT OF THE 